Chelating agents have been used in a variety of applications including textile dyeing, hydrogen peroxide stabilization, pulping, removal of scale deposits, cleaning metal surfaces and detergents. Scale removal generally employs strongly caustic solutions of chelating agents such as ethylenediamine tetraacetic acid (EDTA) or nitrilotriacetic acid (NTA). Thus, for example U.S. Pat. No. 3,951,827 to Burroughs et al discloses scale removing compositions comprising a polyaminocarboxylic acid chelating agent (such as e.g. EDTA or NTA) a synergistic, water-soluble inorganic fluoride salt, and optionally, surfactants, freezing point depressants and buffering agents, and which is adjusted to a pH in the range 8 to 9 by the addition of a suitable base. Phosphonic acid/amino acid chelant mixtures, using high concentrations of each, have also been used to remove rust scale as disclosed in U.S. Pat. No. 3,806,458.
EP 225,015 discloses mixtures of amino acid chelant, organophosphonate and zinc, having respective concentrations of from 0 to 10 ppm, for use as corrosion inhibitors for mild steel. At these low concentrations, the amino acid chelant is not corrosive to the metal surfaces, however, the use of high concentrations of chelating agents often causes undesirable accelerated corrosion rates on the metal surfaces in the treated systems. Various alkyl phosphates have been used to minimize corrosion caused by detergents which contain NTA or EDTA. For example, Tenside Detergents 8(5),237(1971) discloses that the use of monoalkylphosphates and dialkylphosphates combined with fatty alcohols, alkyl polyglycol ethers or paraffins was effective in inhibiting corrosion of metals in contact with a chelate containing solution.
It has now been discovered that small amounts of a particular organophosphonate, namely 1-hydroxyethane-1,1-diphosphonic acid, surprisingly provides enhanced corrosion inhibiting properties in aqueous chelant solutions.